Thick Upper Mantle Transition Zone beneath Taiwan and the Surrounding Region

Details Thick Upper Mantle Transition Zone beneath Taiwan and the Surrounding Region Thick Upper Mantle Transition Zone beneath Taiwan and the Surrounding Region

Dec 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufm.t32b0932h&link_type=abstract

American Geophysical Union, Fall Meeting 2003, abstract #T32B-0932

Physics

3672 Planetary Mineralogy And Petrology (5410), 7207 Core And Mantle, 8124 Earth'S Interior: Composition And State (Old 8105), 8180 Tomography

Scientific paper

The deep mantle structure beneath Taiwan and the surrounding two subduction systems remains poorly understood. In south Taiwan the eastward-subducting Eurasian plate outlined by the Wadati-Benioff zone seismicity disappears to the north of 23oN. Because of the lack of tomographically-imaged velocity structure in deep mantle, it remains unclear whether an aseismic slab extends to greater depths or mantle downwelling occurs as a result of gravitational instability of the thickening lithosphere beneath the Taiwan orogen. One approach to address the problem is to map the thickness of the transition zone from differential times between two P-to-S conversions at the discontinuities near 410 and 660 km depth. The corresponding seismic velocity jumps have been respectively attributed to the transitions from α -phase to β -phase of (Mg,Fe)2SiO4 and from γ -phase to perovskite plus magnesiowustite. Decreasing temperature shoals and depresses the boundaries of the phase transformations near the 410-km and 660-km discontinuities, respectively; thus increases the thickness of the transition zone. Therefore, variations in the discontinuity depths provide constraints on the thermal state in response to cold slabs or downwelling mantle. The data used for the study are receiver functions derived from teleseismic body waves recorded in the broadband BATS network on Taiwan and off-shore islands. Pds (referring to a P-to-S conversion at the depth d) paths with common piercing points are binned together in square patches of 260x260 km. The stacked receiver functions show that P660s-P410s differential times are greater than in the IASP91 model throughout the entire study area, indicating a relatively thicker and colder transition zone. The most anomalous region with a 6-s excess differential time is centered at (22oN, 121.8oE) near Lanyu island, about 250 km east of the southern end of Taiwan. The delays of P660s-P410s times decrease toward the north and the west; only ˜1.5-s delay remains beneath north Taiwan and the western Taiwan Strait. Lateral variations in the thickness and temperature of the transition zone will be explored after correcting the signature associated with upper mantle heterogeniety.

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